Typical examples of materials requiring high surface flatness include a single crystal silicon disk referred to as a silicon wafer for manufacturing semiconductor integrated circuits (IC, LSI). The silicon wafer is required to have a finished flat surface with high accuracy in each of steps of laminating and forming an oxide layer and a metal layer in order to form the reliable semiconductor junction of various thin films used for forming circuits in the steps of producing an IC, LSI, etc. In the polishing and finishing steps, a polishing pad is generally stuck to a rotatable support disk referred to as a platen, and a material to be polished such as a silicon wafer is stuck to a polishing head.
As a method for fixing a material to be polished such as a silicon wafer and glass to a polishing head, there have been conventionally adopted (1) a wax mounting method for fixing a polishing head and a material to be polished via wax, (2) a vacuum chuck method for fixing a material to be polished to a polishing head by suction, and (3) a no-wax mounting method for fixing a material to be polished by an artificial leather or polymer foaming sheet stuck to a polishing head.
In the wax mounting method, there is a problem that the material to be polished is unlikely to be attached and detached, the wax should be removed by cleaning the material to be polished after polishing, and thus the operating steps are complicated since the wax is used.
The vacuum chuck method has a problem that the material to be polished transforms in a suction port part and thereby the surface accuracy of the material to be polished after polishing is deteriorated.
The no-wax mounting method has advantages of easy attachment and detachment of the material to be polished and excellent production efficiency. However, the method has a problem that the surface smoothness of the material to be polished after polishing is also deteriorated when the surface accuracy of the holding surfaces of the artificial leather and polymer foaming sheet is poor.
The use of a backing material which is provided with an adhesive resin layer made of a resin on the surface of a foamed layer has been proposed as a method for solving the problem of the no-wax mounting method in JP-A 2002-355754. A sheet-shaped elastic foam, in which independent cells having an elongated shape and a larger diameter toward the surface of a foamed layer are provided for a foamed layer, has been proposed in JP-A 6-23664.
However, in a process for producing the backing material as presented in JP-A 2002-355754, the operating steps are complicated since the adhesive resin layer are separately provided and an elastic layer is produced by wet coagulation. Also, the process has problems of high environmental burden since a solvent has to be used and necessity of a large amount of water for the extraction of the solvent in the production. It is difficult to form the above cells according to JP-A 6-23664, and furthermore, it is considered that the modification of the foamed layer is increased since the void rate of the surface of the foamed layer is increased, thereby deteriorating the surface smoothness of the material to be polished after polishing. The process has problems that air bite occurs when the material to be polished is stuck to the foamed layer and a water film of slurry occurs between the foamed layer and the material to be polished since the water-absorbing property is low to reduce the adsorptive property of the material to be polished.